The present invention relates generally to testing substrates and more particularly to testing electrically conductive substrates.
In recent years, small "laptop" computers have become popular. Because of their size and portability, small flat panel screens are used in laptop computers instead of more traditional video displays. FIG. 1A shows a laptop computer 10 with a flat panel display 12.
A side view of a typical flat panel display 12 is shown in FIG. 1B. During operation, selected pixels 14 on an active matrix plate are switched on or off. For a color display, the pixels 14 are grouped in triplets corresponding to the colors red, green, and blue. A liquid crystal 18 is sandwiched between the active matrix plate 16 and a color filter plate 20 having color filters 22. Seals 24 at edges of the flat panel display 10 hold the active matrix plate 16, the liquid crystal 18, and the color filter plate 20 together and prevent the liquid crystal 18 from spilling out of the active matrix plate 16.
The pixels 14 which are turned on are set at various voltages. The voltages affect parallel and perpendicular dielectric constants of the liquid crystal 18. By changing these dielectric constants, the liquid crystal's ability to polarize light also changes. Light impinging upon the active matrix plate 16 is indicated by upward arrows in FIG. 1B. During operation, light shines through the active matrix plate 16 and is polarized in the liquid crystal 18. The light is then filtered by the color filters 22 to make an image on the flat panel display 12.
FIG. 1C shows a schematic representation of an active matrix 26 on an active matrix plate 16. Each pixel 14 is controlled by a transistor 28. By applying a voltage to one of an odd set of data electrodes 30 or even set of data electrodes 32, a source side of each transistor 28 can be biased. By applying another voltage to a gate of a selected biased transistor 28 via either odd gate electrodes 33 or even gate electrodes 34, a drain for the selected biased transistor 28 is activated, and a voltage is applied to the corresponding pixel 14. In this way, each pixel 14 in the active matrix plate 16 can be activated.
As with other electrical devices, flat panel displays 12 and the active matrix plates 16 are tested for quality assurance. FIG. 2A is a diagrammatic representation of a side view of a conventional active matrix plate tester 40. Under testing, a probe frame 42 with probe leads 43 sits atop the active matrix plate 16. The probe frame 42 electrically connects a pattern generator 44 to the data electrodes 30, 32 and gate electrodes 33, 34. The pattern generator 44 outputs voltages to the active matrix plate electrodes 30, 32, and 34 to activate the various transistors 28 and pixels 14.
Motors (not shown) horizontally and vertically position a sensing head 46 over a local area of the active matrix plate 16 to be tested. The plate rests on a support 45. Once positioned, other motors (not shown) orient the sensing head 46 with respect to an upper surface 48 of the active matrix plate 16. Similarly, a half-silvered mirror 50 is horizontally and vertically positioned and oriented with respect to the sensing head 46.
During testing, the pattern generator 44 attempts to activate a specific pixel or pixels 14 by sending voltages to the active matrix plate 16 as described above and electrically biasing a rigid sensing material 47 on the sensing head 46. The rigid sensing material 47 responds to voltage changes generated by the pixels 14 by changing its optical properties. The pattern generator 44 commands a light source 51 to shine a beam of light onto the half-silvered mirror 50. The half-silvered mirror 50 reflects the beam through the rigid sensing material 47 and sensing head 46. The light then travels down and is reflected back up by a mirrored bottom of the rigid sensing material 47. The reflected light then passes upward through the sensing head 46 and the half-silvered mirror 50.
A camera 52 photographs the reflected light. The pattern generator 44 directs the camera 52 to photograph the light once the sensing head 46 and the half-silvered mirror 50 are positioned and oriented and after the light source has emitted its beam of light.
Since the sensing head 46 can only be in proximity to part of the active matrix plate 16, the conventional active matrix plate tester 40 must make several iterations of the above procedure resulting in several pictures of local areas of the active matrix plate 16. Either before or after the camera 52 takes all the pictures, each picture is transferred into a digital format. Because the rigid sensing material 47 provides relatively weak optical signature, image processing is required to extract defect information from the digitized pictures. Therefore, an image processor 54 must process the pictures before defect patterns can be shown on a monitor 56.
FIG. 2B is a diagrammatic representation of a sensing head 46 while testing a local area of an active matrix plate 16. Generally, the rigid sensing material is glued to the sensing head 46. Because the sensing material 47 is rigid, the sensing material does not contact the surface of the active matrix plate 16 because the rigid sensing material 47 may damage the active matrix plate 16. Furthermore, the surface irregularities create irregular distances between the top surface of the active matrix plate 16 and the rigid sensing material 47. Thus, the sensitivity of the rigid sensing material 47 to pixel voltages is drastically degraded. Largely because of this, the pictures taken by the camera 52 must undergo image processing as described above.
FIG. 3 is a flow diagram summarizing a typical conventional method 60 of testing an active matrix plate 16 beginning at a step 62. This conventional method 60 was just described with reference to the conventional active matrix plate tester 40 in FIG. 2A. Initially, the active matrix plate 16 is placed in the tester 40 in a step 64. Then, the probe frame 42 is positioned on the active matrix plate 16 in a step 66. The sensing head 46 with the rigid sensing material 47 is then placed over a local area to be tested in a step 68 and the sensing head 46 and rigid sensing material 47 are oriented with respect to the local area in a step 70. In a step 72, the pattern generator 44 applies voltages to the active matrix plate 16 via data electrodes 30, 32 and gate electrodes 33,34. Step 74 applies a voltage to bias the rigid sensing material 47 at an operating voltage. As with the light source 52 and half-silvered mirror 50 as discussed above, the local area is illuminated in a step 76. After being reflected off the bottom of the rigid sensing material 47 and passing through the rigid sensing material 47, the camera detects the light in step 78. The camera pictures are image processed in step 80 to remove noise.
Step 82 determines whether all test patterns for a particular voltage have been sent to the active matrix plate 16. Generally, a voltage signal is applied to the odd electrodes 30 followed by the same signal to the odd electrodes 30 with negative voltage magnitude. Step 82 determines that not all signals have been applied after the positive and negative voltage signals have both been applied to the odd electrodes 30. Then the same signals are "reversed" in a step 84 by applying them to even electrodes 32. Steps 76, 78, and 80 are then repeated for the positive and negative voltage signals applied to the even electrodes 32. Then step 82 answers yes, and step 86 determines whether all local areas have been tested. If not, conventional method 60 returns to step 68 to repeat steps 68, 70, 72, 74, 76, 78, 80, 82, and 84 for a new local area. Once all local areas have been tested, step 86 passes to step 88 where an image Indicating defects in the active matrix plate 16 are displayed. Conventional method 60 ends at step 90.
The conventional method 60 has several drawbacks. First, only a relatively small part of the matrix plate 16, or other substrate, can be tested at one time. For this reason, complex and cumbersome systems of motors are required to position and to orient the sensing head 46, the half-silvered mirror 50, the camera 52, or a support on which the plate 16 or substrate rests. Also, multiple steps are required to position and to orient the equipment over each local area.
Additionally, the gap between the sensing head material 47 and the top surface of the active matrix plate varies. Consequently, the amount of noise varies from local area to local area. It also varies within each local area as the gap between the rigid sensing material 47 and top surface 25 of the matrix plate 16 varies.